Photosensitive element containing UV sensitive terpolymers

ABSTRACT

A photolithographic resist with excellent sensitivity for actinic radiation in the short wavelength ultraviolet region is produced from terpolymers of (1) methyl methacrylate, (2) materials such as 3-oximino-2-butanone methacrylate, and (3) compounds such as methacrylonitrile.

This is a division of application Ser. No. 236,114, filed Feb. 19, 1981,now U.S. Pat. No. 4,343,889.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to lithography and, more particularly, tophotolithography.

2. Art Background

A variety of lithographic processes are presently used in thefabrication of semiconductor devices. In particular, a commonlithographic technique is to employ light to expose an organic polymerthat has been coated on a substrate such as a silicon wafer. The polymeris formulated so that the regions exposed to light undergo a chemicalreaction. For positive resists, the chemical reaction results in acompound in the exposed regions that has, in an appropriate solvent, asignificantly greater solubility than the original polymer. The patterndelineated in the polymer is developed by using this solvent to dissolvethe exposed regions while leaving essentially unaffected the portions ofthe resist that have not been subjected to actinic radiation.

The resolution attained is affected by the wavelength of the exposingradiation. Generally, the shorter the wavelength of the exposingradiation, the better the resolution that is ultimately achievable.Therefore, extensive research has centered on the formulation of resiststhat are suitable for exposure by short wavelength ultraviolet light,i.e., by light in the wavelength range between 200 nm to 300 nm.

Generally, for a resist material to be appropriate for ultravioletlithography, it should have a relatively high sensitivity to ultravioletradiation, i.e., should require less than 200 millijoules per cm² ofincident short wavelength ultraviolet light to form the desired image.The requisite of high sensitivities is necessitated by practicalconsideration. Generally, useful sources of short wavelength ultravioletradiation generate a relatively low intensity. Thus, to attain adequateexposure during a time period acceptable for most applications, amaterial with relatively high sensitivity to ultraviolet radiationshould be utilized.

A variety of attempts have been made to produce materials that arepatternable by exposure to ultraviolet radiation. For example, U.S. Pat.No. 3,558,311, issued Jan. 26, 1971, describes the solution degradationof a few copolymers of methyl methacrylate and 3-oximino-2-butanonemethacrylate (OBM). Additionally, poly(methyl methacrylate) (PMMA) isalso known to absorb in the short wavelength ultraviolet. These polymershave a non-negligible absorption coefficient in the ultraviolet region.However, the sensitivity of PMMA to short wavelength ultravioletradiation is unacceptable and the liquid phase experiments done with thecopolymers yielded no information concerning the solid phasecharacteristics such as sensitivity and resolution.

The absence of disclosures concerning a sensitive UV resist isindicative of the difficulty in developing such a resist. Thesensitivity of a resist to a given exposing radiation depends on thechange in solubility produced in the exposed region of the film throughan induced chemical reaction. Although certain organic functional groupsare known to absorb in the UV, the resulting chemical transformation(s)in the solid state of molecules having these groups is a complex anduncertain matter. Therefore, although generally it is determinablewhether a material will absorb in the UV, it is not predictable how thecompound will be altered or what the change in solubility will be as aresult of this absorption. Thus, resists absorbing short wavelengthultraviolet radiation have been developed, but resists with relativelyhigh sensitivities are typically not available.

SUMMARY OF THE INVENTION

Photosensitive bodies based on resists that absorb in the UV and thathave a sensitivity to short wavelength UV light up to 20 millijoules percm² at 240 nm are producible. The resists include terpolymers formedfrom a composition such as methyl methacrylate that is polymerized withan OBM based compound and with an acrylonitrile based compound. Forexample, a polymer that is formed from 79% methyl methacrylate, 6% OBM,and 15% methacrylonitrile exhibits a sensitivity approximately 10 timesgreater than poly(methyl methacrylate) at 240 nm while allowing theformation of patterns with dimensions as small as 1 μm. Threefoldimprovements are also achieved over corresponding copolymers of methylmethacrylate and OBM without the methacrylonitrile. The UV sensitivebodies are produced by coating the resist on a substrate, such as asilicon wafer. It is then possible to delineate the resist in a patternsuitable for use in device fabrication with practical exposure timeswith commonly available sources.

DETAILED DESCRIPTION

Photosensitive bodies are produced by coating a resist material onto asubstrate such as a semiconductor body, e.g., a silicon wafer. Thiscoating is done by dissolving a photosensitive material such as aphotosensitive polymer into a suitable solvent which is employed todeposit the polymer onto a substrate by conventional techniques such asspinning. In spinning, the dissolved polymer is coated onto the waferand the wafer is then rotated. This procedure results in a relativelyuniform layer of the polymer. (See DeForest, Photoresist Materials andProcesses, page 223, 1975, McGraw-Hill, for a general description of thespinning procedure.)

By using a specific class of polymers, a photosensitive body, i.e.,bodies with sensitivities up to 20 millijoules per cm² at 240 nm areproducible. This class of materials encompasses polymers formed fromthree monomers--(1) a methacrylate ester compound, i.e., ##STR1## whereX₁ is an alkyl, preferably a methyl, (2) an OBM based compound, i.e.,##STR2## where R₁ is methyl, ethyl, or propyl, R₂ is methyl, ethyl, orpropyl, or R₁ and R₂ are connected to form 5 or 6 member carbocylicrings, such as ##STR3## and (3) an acrylonitrile based compound, i.e.,##STR4## where X₂ is CH₃ or Cl. The X₁ group strongly affects the glasstransition temperature of the final polymer. An X₁ that yields a polymerthat is too soft, i.e., a polymer with a glass transition temperaturebelow 90 degrees C., limits the dimensional stability and, thus, theresolution. For typical monomer combinations, X₁ is preferably chosen tobe a methyl radical.

The desired polymers are prepared from the monomers by conventionalpolymerization techniques. For example, the monomers (together with acatalyst if used) are mixed in a solvent, e.g., toluene, ethyl acetate,or acetone. The mixture is then agitated at a temperature in the range60 to 130 degrees C. Temperatures higher than 130 degrees C. generallyinduce excessive decomposition of the OBM monomer. Temperatures lowerthan 60 degrees C. are employable and are not precluded. However, lowertemperatures require longer reaction times. The reaction is typicallystopped after 20 to 30% of the monomers have reacted. This limitation isdone to ensure a narrow molecular weight dispersivity. This degree ofcompletion generally occurs in a time period in the range 4 to 24 hoursfor temperatures in the range of 60 to 130 degrees C.

The resolution and sensitivity obtained depend on the average molecularweight (defined by Billmeyer, Textbook of Polymer Science, page 6, 1971J. Wiley & Sons) of the polymer, the distribution of the molecularweight, and the relative proportions of the monomers in the polymer. Forsensitivities better than 200 millijoules per cm², it is generallydesirable to employ polymers with molecular weights greater than 50,000,preferably greater than 100,000 and dispersivities below 3.5, preferablybelow 3. (Dispersivities are defined in Billmeyer, Supra, page 6.)Molecular weights, however, that are larger than 2,000,000 are typicallynot desirable because the resulting polymer solution used in coating isexcessively viscous.

The molecular weight of the polymer is determined by the polymerizationreaction temperature, the catalyst, if any, the catalyst concentration,as well as the concentrations of the three monomers that are utilized.These parameters are interrelated and a control sample is utilized todetermine the specific conditions necessary to yield a desired molecularweight. However, generally for molecular weight in the desired range,temperatures in the range 60 to 130 degrees C., with free radicalinitiators such as benzoyl peroxide in a concentration range from 0 to10⁻³ M, and monomers of methyl methacrylate compounds, OBM compounds andacrylonitrile compounds having concentrations in the range 0.5 to 3 M,0.1 to 1 M, and 0.05 to 1 M, respectively, are employed.

Dispersivity is predominantly dependent on the reaction temperature, thecatalyst, if any, and the percentage of the monomers converted into thepolymer. To obtain dispersivities in the desired range, the parameterranges previously discussed in the context of molecular weight andreaction time are employed. A control sample is used to determine theprecise conditions needed for a particular desired dispersivity.

As discussed, sensitivity also depends on the relative proportions ofthe monomers incorporated into the polymer. This proportion isessentially equal to the relative proportion of monomers used in thereaction mixture. (There is a difference between the amount of2-chloroacrylonitrile in the reaction mixture and in the final polymer.A control sample is used to determine the precise amount of reactantsneeded to obtain the desired polymer composition given below when2-chloroacrylonitrile is employed.) Generally, to attain a desirablesensitivity and resolution the mole percent in the final polymer ofacrylonitrile compound should be in the range 10 to 30%, preferably 15to 25%, the mole percent of OBM compound should be in the range 3 to30%, preferably 9 to 20%, and the remainder should be methylmethacrylate compound. (The addition of other monomers to the threemonomers used to form the terpolymer is possible. However, theproportions of the three monomers should not be altered.)

It is possible to mix various additives with the subject polymers beforecoating the polymers onto a suitable substrate. For example, to increasethe sensitivity of the polymers to short wavelength ultravioletradiation, photosensitizers capable of absorbing UV radiation andtransferring energy to the OBM moiety, such as p-t-butylbenzoic acid,are employed. For the subject polymers singlet sensitizers arepreferably used, i.e., sensitizers that rely on energy transferred froman excited singlet state are employed. The sensitizers are preferablychosen to absorb in the wavelength region in the short wavelengthultraviolet where the exposing source has useful intensity.

As discussed, the subject photosensitive bodies are useful for obtainingrelatively short exposure times for processes that delineate a patternwith short wavelength UV radiation. In such a process, the subjectphotosensitive bodies are exposed, for example, by directing UVradiation through a suitable mask. Typically, resist thicknesses in therange 0.3 to 1.5 μm are employed. Exposure is continued for a periodsufficient to allow, upon development, complete removal of theirradiated regions of the resist layer. In this regard, it is generallydesirable that the optical density of the photosensitive polymer on thesubstrate is not greater than 0.5. Although larger optical densities arenot precluded such densities hinder the exposure of the polymer throughits thickness since the exposing radiation is predominately absorbednear the surface. To obtain the preferred optical densities, the ratioof monomers discussed in connection with sensitivities is used. Afterexposure, the resist is developed by treating the photosensitive bodywith a suitable solvent such as methyl isobutyl ketone or a mixture ofmethyl isobutyl ketone with 2-propanol. Development is generallycomplete in a time period of 30 to 120 seconds. Development should notbe so long as to significantly thin the unirradiated portions of theresist.

The following examples are illustrative of resists and exposureprocesses within the subject invention and compare the subject inventionwith other photosensitive bodies:

EXAMPLE 1 Preparation of 3-oximino-2-butanone methacrylate (OBM)

The monomer 3-oximino-2-butanone methacrylate (OBM) was prepared. Thispreparation began by cooling to a temperature of 0 degree C. with an icewater bath, a solution of butanedione monoxime (54.5 g) andtriethylamine (120 ml) in ether (1000 ml). Approximately 56 ml ofmethacryloyl chloride in 250 ml of ether was added dropwise under anitrogen atmosphere over the course of 1 hour. The solution was stirredduring this addition and the stirring was continued under nitrogen at 0degree C. for 3 hours after the addition was completed. The resultingmixture was filtered to remove the solid residues. The solid residueswere discarded, and the liquid obtained was transferred to a separatoryfunnel. Approximately 300 ml of water was added to the separatory funneland the liquid was agitated. The water and ether phases were separated,the water phase was discarded, and the ether phase was dried overmagnesium sulfate for 1 hour. The magnesium sulfate was removed byfiltration and the ether solution was evaporated to dryness on a rotaryevaporator using water aspirator vacuum. The resulting solid wasrecrystallized from cyclohexane to yield 45 g of OBM. (The materialobtained melted between 38.5 and 39 degrees C.)

EXAMPLE 2 The Preparation of Polymers

Poly(methyl methacrylate) (PMMA) was utilized as obtained from DuPont.This polymer was denominated Elvacite 2010 by DuPont and was a polymerhaving an average molecular weight of approximately 120,000.

Copolymers of methyl methacrylate and OBM were synthesized. A variety ofthese copolymers, as listed in Table I, were produced. The followingprocedure was the same for each copolymer except the relative amounts ofthe reactants, methyl methacrylate and OBM, were employed as indicatedin the table. The following description is for the polymer whichcontained 87 M percent methyl methacrylate and 13 M percent OBM.

Methyl methacrylate (160 g) and OBM (40 g) were dissolved inapproximately 660 ml of toluene. The solution was heated to 60 degreesC. under an argon atmosphere. This argon atmosphere was produced bybubbling argon into the solution. After approximately 1 hour of bubblingargon through the solution, 34 mg of azobisisobutyronitrile was added.The argon atmosphere and the 60 degrees C. temperature were maintainedfor 24 hours. The argon bubbling and the heating were then terminated.The solution was allowed to cool and then added dropwise to methanolunder agitation to precipitate the polymer. This precipitated polymerwas separated by filtration. The polymer obtained was dissolved in ethylacetate and reprecipitated by again adding the solution to methanol andagitating. The polymer was separated by filtration, and air dried.

EXAMPLE 3 Preparation of Terpolymers

The terpolymers were prepared by the same procedure as described for thecopolymers. Table I indicates relative ratios of the monomers used inthe reactant mixture to produce the various polymers. The followingdescription illustrates the preparation of one of these polymers. Theother polymers listed in the table were prepared in the same mannerexcept the proportions of reactants utilized were as indicated in TableI. (In Table I, M refers to methyl methacrylate, OBM refers to3-oximino-2-butanone methacrylate, and CN refers to methacrylonitrile.)

The reactants employed to produce a terpolymer of 69 M% methylmethacrylate, 16 M% OBM, and 15 M% methacrylonitrile were prepared byfirst producing an ethyl acetate solution of 130 g of methylmethacrylate, 50 g of OBM, and 20 g of methyacrylonitrile in 400 ml ofethyl acetate. (The same preparation is possible in a similar amount oftoluene.) As described in Example 2, after an hour of agitation, 40.4 mgof benzoyl peroxide was added to the solution and the reactants weremaintained at 85 degrees C. for 8 hours. The resulting terpolymer wasthen separated and purified as described in Example 2.

EXAMPLE 4 Preparation and Exposure of Photosensitive Bodies

The polymers listed in Table I were dissolved in methoxyethyl acetateand coated on a substrate and exposed. The sensitivities listed in TableI were obtained by imaging a 1 mm wide slit illuminated by a 1000 Wmercury lamp focussed through quartz condenser optics onto thesubstrate. A series of exposures through this slit onto the substratewere made with each exposure being approximately 1 second longer thanthe previous one. The sensitivity of the film was assigned to theshortest term exposure that allowed removal of the entire thickness ofthe coating upon development without significantly thinning theunexposed portion. The absolute sensitivity of some of the polymers, asindicated in Table I, was determined by isolating the 240 nm line from a1000 W mercury-xenon lamp with a monochromator (resolution of + or -2nm). This line was focussed onto a 1 mm diameter circular contact mask.The same procedure as used to determine relative sensitivities wasfollowed for determining absolute sensitivities. Average molecularweight (M_(w)) and dispersivities (M_(w) /M_(n)) were measured by gelpermeation chromotography and glass transition temperature (T_(g)) wasmeasured by differential thermal analysis.

In each case, the polymer was coated on either a 2 inch or 3 inchcircular substrate. Either a silicon or a silicon dioxide coated siliconsubstrate was employed. The polymer to be coated was dissolved inmethoxyethyl acetate to form a 10% solution. Approximately 2 ml for the2 inch wafers or 3 ml for the 3 inch wafers was placed onto thesubstrate. The substrate was then spun at 3000 rpm for 45 seconds. Theresulting coated substrate was baked at 120 degrees C. for 1 hour in anair oven. After exposure as described above, the photosensitive body wasdeveloped. This development was done by immersing the exposed body in anappropriate solvent. Methyl isobutyl ketone was employed for thecopolymer, a mixture of methyl isobutyl ketone and 2-propanol (80/20)was employed for the terpolymers, and methyl isobutyl ketone wasutilized for the poly(methyl methacrylate). Immersion in a solvent wasdone for approximately 1 minute and then the photosensitive bodies weredried in air at room temperature.

                                      TABLE I                                     __________________________________________________________________________                                      Relative Exposure                                     Molar  M.sub.w          Required.sup.1                              Polymer   Composition                                                                          (× 10 + 5)                                                                    Mw/Mn                                                                              Tg(-°C.)                                                                     200-400 nm                                  __________________________________________________________________________    PMMA.sup.c                                                                               --    --    --   105   1(3400)                                     GROUP I                                                                       P(M-OBM)  97:3   1.39  2.06 103   0.40                                        P(M-OBM)  94:6   2.87  1.86 103   0.25(800)                                   P(M-OBM)  91:9   6.50  2.17 --    0.25(700)                                   P(M-OBM)  87:13  2.05  2.09 --    0.025                                       P(M-OBM)  84:16  2.48  2.04 95    0.033(150)                                  P(M-OBM)  63:37  1.88  2.35 89    0.02                                        GROUP II                                                                      P(M-OBM-CN)                                                                             86:6:8 1.82  2.60 103   0.25                                        P(M-OBM-CN)                                                                             79:6:15                                                                              3.41  2.66 99    0.1                                         P(M-OBM-CN)                                                                             73:6:22                                                                              2.70  2.53 --    0.1                                         P(M-OBM-CN)                                                                             66:6:28                                                                              1.40  2.30 --    0.2                                         GROUP III                                                                     P(M-OBM-CN)                                                                             82:3:15                                                                              2.06  1.84 100   0.33                                        P(M-OBM-CN)                                                                             79:6:15                                                                              3.41  2.66 99    0.1(300)                                    P(M-OBM-CN)                                                                             76:9:15                                                                              2.27  2.19 98    0.017                                       P(M-OBM-CN)                                                                             73:12:15                                                                             2.97  1.92 95    0.017                                       P(M-OBM-CN)                                                                             69:16:15                                                                             2.78  1.98 96    0.012(40)                                   P(M-OBM-ClCN).sup.2                                                                     (70:25:5)                                                                            --    --   --    0.012                                       __________________________________________________________________________     .sup.1 () =  absolute sensitivity as measured at 240 nm in mJ/cm.sup.2        .sup.2 Terpolymer of methyl methacrylate  OBM and chloroacrylonitile     

EXAMPLE 5 Preparation and Exposure of Polymers with Sensitizers

The polymers indicated in Table II were prepared as described in Example3. As indicated, compositions both with and without a sensitizer wereproduced. The compositions with a sensitizer were made as described inExample 4, except after purification of the polymer and upon preparationof a solution for spinning approximately 15 weight percent ofp-t-butylbenzoic acid was added. The remaining procedure, includingexposure and development, was the same.

                  TABLE II                                                        ______________________________________                                        Polymer    Composition                                                                              % Sensitizers                                                                              Sensitivity.sup.1                          ______________________________________                                        P(M-OBM-CN)                                                                              86:6:8     --           0.25                                       P(M-OBM-CN)                                                                              86:6:8     15           0.017                                      P(M-OBM-CN)                                                                              79:6:15    --           0.1                                        P(M-OBM-CN)                                                                              79:6:15    15           0.008                                      P(M-OBM-CN)                                                                              73:6:22    --           0.1                                        P(M-OBM-CN)                                                                              73:6:22    15           0.012                                      P(M-OBM-CN)                                                                              66:6:28    --           0.2                                        P(M-OBM-CN)                                                                              66:6:28    15           0.033                                      ______________________________________                                         .sup.1 Exposure required relative to PMMA, where PMMA = 1                

What is claimed is:
 1. A photosensitive body comprising a substrate incontact with a photosensitive material comprising an organic polymercharacterized in that said polymer is formed from the monomerscomprising (1) a methacrylate ester compound represented by the formula##STR5## where X₁ is an alkyl, (2) a 3-oximino-2-butanone methacrylatebased compound represented by the formula ##STR6## where R₁ is chosenfrom the group consisting of methyl, ethyl, and propyl, R₂ is chosenfrom the group consisting of methyl, ethyl and propyl or R₁ and R₂ areconnected to form a 5 or 6 member ring, and (3) an acrylonitrile basedcompound represented by the formula ##STR7## where X₂ is CH₃ or Cl. 2.The photosensitive body of claim 1 wherein said substrate comprisessilicon.
 3. The photosensitive body of claim 1 wherein saidphotosensitive material includes a sensitizer.
 4. The photosensitivebody of claim 3 wherein said sensitizer comprises p-t-butylbenzoic acid.5. The photosensitive body of claim 1 wherein said polymer has amolecular weight in the range 50,000 to 2,000,000.
 6. The photosensitivebody of claim 5 wherein said molecular weight is in the range 100,000 to2,000,000.
 7. The photosensitive body of claim 1 wherein X₁ is methyl.8. The photosensitive body of claim 1 wherein said polymer has a molepercent of said acrylonitrile compound in the range 10 to 30% and a molepercent of said 3-oximino-2-butanone methacrylate compound in the range3 to 30%.
 9. The photosensitive body of claim 8 wherein said molepercent of said acrylonitrile compound is in the range 15 to 25%. 10.The photosensitive body of claim 8 wherein said mole percent of said3-oximino-2-butanone methacrylate compound is in the range 9 to 20%.